1. Field of the Invention
The present invention relates to a cellular communication system using distributed antennas, and more particularly to an algorithm for determining a link combination of a Mobile Station (MS) with a Base Station (BS) or a Relay Station (RS), and a method for allocating resources based on the determined link combination, in a Distributed Antenna System (DAS) where the BS is connected to the Relay Stations by wire or a dedicated line.
2. Description of the Related Art
In the case of a conventional cellular communication system, single-hop scheme and multi hop schemes have been widely used. In the single-hop scheme, one BS exists within a cell and transmits a signal to MSs within a cell coverage area. In the multi-hop scheme, one BS and multiple RSs are arranged within a cell, and the one BS is wirelessly connected to the multiple RSs. In the case of a multi-hop system using multiple wireless RSs partly sharing the wireless resources with the BS, a transmission area is reduced over that of an existing cellular system where only a BS exists, so that it is possible to reduce transmission power. Also, shortening of transmission distance between the RS and an MS causes reduced path loss, so that it possible to transmit data at a higher speed. Accordingly, transmission capacity of the cellular system can be increased.
However, in the case of the multi-hop system utilizing a wireless RS, the transmission for data relay is additionally required, as compared with a single-hop network, and at this time, several relay links must share limited resources. This sharing therefore can deteriorate user service quality. Namely, a wireless RS system can improve a received Signal to Interference plus Noise Ratio (SINR) regarding each MS in an area outside of a cell, but using a part of frame resources for relay transmission causes the reduction of effective channel resources that can be allocated to each MS. Thus, it is hard to significantly increase system transmission capacity. Therefore, in an attempt to overcome the limit of the wireless RS system where the same resources must be transmitted several times from the BS to the RS, studies on a wired RS system where a cell is configured in such a manner that a link between a BS and an RS is connected by an optical cable, are being actively conducted.
A basic configuration of a network of a wired RS system is the same as that of the wireless RS system, but is different in that a link between the BS and the RS or a link between a first RS and a second RS is connected by wire and a fixed RS is used. In the wired RS system, cost is a factor in installing an optical cable between the BS and the RS, and the RS is hard to move after it is installed. However, a wired connection between the BS and the RS causes no signal loss, and interference is reduced as compared with the wireless RS system. Also, by transmitting various kinds of control signals to a wired section between the BS and the RS, a resource allocation technique and a signal combing scheme between RSs, which are restricted to the wireless RS system, can be applied to the wired RS system.
A signal combining technique is largely classified into a microscopic diversity scheme and a macroscopic diversity scheme. In the microscopic diversity scheme, a receiving end or a transmitting end combines signals by using multiple antennas. In the macroscopic diversity scheme, two or more BSs or RSs transmit signals, and each combination between the signals is utilized.
Typical examples of the microscopic diversity scheme include a Selection Combining (SC) scheme for selecting a signal of the best quality among signals received through multiple receiving antennas, a Maximum Ratio Combining (MRC) scheme for maximizing a received signal-to-noise ratio during signal combining, an Equal Gain Combining scheme for matching a phase between received signals and then combining signals having the matched phase, etc.
The macroscopic diversity scheme is different from the microscopic diversity scheme only because a signal combining scheme is applied to signals from two or more BSs or RSs that are geographically remote. The macroscopic diversity scheme uses a Macroscopic Diversity Combining (MDC) scheme utilizing several microscopic diversity schemes based on compensation techniques for performance degradation factors. Such factors are, for example, the difference between time delays of received signals, etc., during signal combining due to geographical characteristics. The microscopic diversity scheme can overcome short-term fading due to multi-path by using multiple antennas to combine signals at transmitting/receiving ends. However, since the microscopic diversity scheme has a limit on improving channel quality due to overcoming long term fading caused by topographical obstacles, the macroscopic diversity scheme is used in order to overcome the long-term fading.
In the macroscopic diversity scheme, signals transmitted from multiple RSs are combined together. Accordingly, the macroscopic diversity scheme can effectively overcome the long-term fading due to topographical obstacles. However, in the macroscopic diversity scheme, the multiple RSs allocate the same frequency resources for signal combining, so that frequency efficiency is reduced due to consuming additional resources as compared with signal transmission by a single RS. Nevertheless, by finding a condition for determining signal combining which offsets the reduced frequency efficiency and enlarges transmission capacity through signal combining, and then allocating resources based on the found condition for determining signal combining, it is possible to significantly improve average frequency efficiency over the signal transmission by a single RS.
Studies have been conducted on hand-off techniques for switching between channels and connecting a relevant communication to a switched channel when, conventionally, one MS moves from a first communication zone of a particular BS to a second communication zone of another BS based on the techniques as described above. A hand-off scheme is largely classified into a hard hand-off scheme and a soft hand-off scheme. In the hard hand-off scheme, an existing channel is disconnected, and a new channel is then connected. In the soft hand-off scheme, an existing channel is disconnected in a state where the existing channel and a new channel are simultaneously connected.
The latter soft hand-off scheme is employed by a conventional cellular system, and the existing soft hand-off scheme uses an SINR-based condition for determining signal combining. Depending on the SINR-based condition for determining signal combining, if a first signal strength regarding an MS from a new cell increases above a particular value relative to a second signal strength regarding the MS from a cell adjacent to the MS, the conventional cellular system employing the existing soft hand-off scheme enters a signal combining mode. Also, depending on the SINR-based condition for determining signal combining, if a signal strength regarding the MS from an existing cell decreases below the particular value, the conventional cellular system employing the existing soft hand-off scheme operates in a single transmission mode related to the new cell again. The existing soft hand-off scheme improves channel quality regarding an MS moving to a boundary area among cells, and prevents a communication disabled state regarding the MS. Accordingly, the existing soft hand-off scheme enables a stable communication satisfying volume required for minimum transmission of a user regardless of geographical conditions within each cell.
However, in terms of the entire cell, the existing soft hand-off scheme doesn't ensure a maximum system transmission capacity Namely, a condition for determining signal combining for the maximum transmission capacity which can be expected during the utilization of the signal combining only performs a threshold value test with SINR values during signal transmission from an adjacent cell. Therefore, the condition for determining signal combining doesn't ensure the maximum system transmission capacity, and has raised problems in performing high-complex and high efficient resource allocation.